1. Field of the Invention
The present invention generally relates to a focusing device of a camera, and particularly to a focusing device of a camera having an objective lens which includes one or more lens elements made of a synthetic resin material
2. Description of the Prior Art
It has been a common practice in the photographic camera industry to use one or more lens elements made of a synthetic resin material in an effort to decrease the production costs. The lens element made of a synthetic resin material is called "plastic lens element" hereinafter.
However, the plastic lens element has a drawback in which the refractive power thereof is undesirably changed in accordance with the change in temperature thereof. It causes an undesirable change of a back focal length of a whole lens system including the plastic lens element. The change of the back focal length of the whole lens system causes the deviation of the image plane of the lens system from a predetermined focal plane.
Thus, a plurality of methods are considered for correcting or compensating the change of the back focal length of the lens system including the plastic lens. One of such methods is to control the change of the back focal length of the lens system due to the temperature change within a permissible range The other thereof is to shift at least a part of the lens system for compensating the change due to the temperature change. Further, other thereof is to compensate the change of the back focal length of the lens system by an automatic focusing device provided in the camera body.
Japanese Laid-Open Patent Application No. 111,223/1987 discloses to compensate the change of the back focal length of the objective lens system due to the temperature change by detecting a temperature of an optical system provided in an automatic focus detection device Japanese Laid-Open Patent Application No. 160,129/1984 discloses to change a base line length of a range finder device provided in the camera for compensating the change of the back focal length. Japanese Laid-Open Patent Application No. 64,204/1982 discloses to modify a distance signal indicative of a distance to an object in accordance with a temperature signal produced, for example, by a temperature detector, for compensating the change of the back focal length